patch 8.2.1616: Vim9: cannot pass "true" to synID()

Problem:    Vim9: cannot pass "true" to synID().
Solution:   Use tv_get_bool_chk(). (closes #6860)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 24377b2..ff2379a 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1551,6 +1551,13 @@
   assert_equal(1, searchdecl('blah', true, true))
 enddef
 
+def Test_synID()
+  new
+  setline(1, "text")
+  assert_equal(0, synID(1, 1, true))
+  bwipe!
+enddef
+
 def Fibonacci(n: number): number
   if n < 2
     return n